1. Field of the Invention
The present invention relates generally to lighting applications, and more particularly to a light source comprising an organic light emitting device combined with a layer of photoluminescent material which may have a relatively large area.
2. Description of the Related Art
There are many examples of lighting devices which comprise inorganic light emitting diodes (LEDs) or organic light emitting devices (OLEDs). One example is the commercially available blue inorganic gallium nitride (GaN) LEDs which are coated with phosphor particles to produce white light. These LEDs are essentially point sources of light rather than extended sources of light.
An example of an OLED is set forth in U.S. Pat. No. 5,294,870, which describes an organic electroluminescent multicolor display device comprising an organic electroluminescent source emitting blue light with green-and red-emitting fluorescent materials applied to different subpixel areas. This device emits different colors from the different subpixel areas by color shifting with the green- and red-emitting fluorescent materials.
Another example of an OLED is described in Junji Kido et al., xe2x80x9cMultilayer White Light-Emitting Organic Electroluminescent Devicexe2x80x9d, 267 Science 1332-1334 (1995). This device includes three emitter layers with different carrier transport properties, each emitting blue, green, or red light, which layers are used to generate white light. In this device, however, the layers emitting the different colors typically degrade over time at different rates. Consequently, the color of the device is likely to change over time. In addition, the uniformity of the light output over the emitting area of the device may be less than desirable.
A light source, according to one aspect of the invention, comprises an organic light emitting device which emits light having a first spectrum and a layer of photoluminescent material which absorbs a portion of the light emitted by the organic light emitting layer and which emits light having a second spectrum. The layer of photoluminescent material, which may comprise an inorganic phosphor, typically absorbs less than all of the light emitted by the organic light emitting layer. The light emitted by the organic light emitting layer is mixed with the light emitted by the photoluminescent material to produce light having a third spectrum.
Exemplary embodiments of the invention provide advantages over known devices. For example, if a phosphor is used as the photoluminescent layer, the light emitted from the organic light emitting device is scattered, which provides improved uniformity in light output over the area of the light source. Also, because most phosphors are relatively stable over time, the light source according to exemplary embodiments of the invention has good color stability over time.